1. Field of the Invention
The present invention relates to apparatus and methods used in conducting endothermic reactions and, in particular, to catalytic reforming apparatus and methods for reforming light hydrocarbons and especially mixtures of steam and/or carbon dioxide and light hydrocarbons.
2. Description of the Prior Art
Reforming apparatus and methods heretofore have been used to conduct a variety of endothermic reactions. For example, steam reforming is an established process for converting natural gas and other hydrocarbons into synthesis gas which is commonly referred to as syngas. Another endothermic reaction is the conversion (pyrolysis) of ethane to ethylene which also produces the by-product hydrogen. Depending upon the endothermic reaction, a catalyst may or may not be needed. A catalyst useful for steam reforming of hydrocarbons is nickel on an inert support, such as alumina.
Many commercial steam reformers are of a radiant-type that employs catalyst-filled reaction tubes. These reformers are indirectly heated with a relatively small number of large burners fed by fuel gas and low-pressure air. In the hottest section, heat transfer is primarily by gas radiation, supplemented by refractory radiation and convection. Uniform radiation to the reaction tubes has been reported to be critical since excessive local heating of a reaction tube wall will result in diminished life expectancy for the reformer tubes. To avoid excessive localized heating, a solution has been to provide large spacings between the reaction tubes, the furnace walls, and the burner flames. Consequently, these radiant reformers have been large and expensive on a per unit of capacity basis. They also usually require additional heat exchangers to cool the syngas product and to preheat the feed gas. Generally, the operational principles associated with these prior art radiant reformers do not lend themselves to a relatively low cost, efficient and compact design coupled with long reaction tube life expectancy.
Various attempts have been made to provide an endothermic reaction apparatus that achieves the objects of compact design and long life expectancy. In U.S. Pat. No. 4,692,306, these objects are said to be accomplished by a novel burner chamber configuration which prevents flame impingement or direct line-of-sight between burner flame and reaction chamber.
Accordingly, much conventional thinking has been directed away from direct flame impingement on reaction tubes in prior art endothermic reaction apparatus, primarily because of the excessive localized heating of the reaction tubes that arises from direct flame impingement using conventional burner designs.